At many points in the manufacture of semiconductor chips, a semiconductor wafer is etched in a pattern defined by the apertures of an overlying mask. Plasma etching of several of the layers of thin films on semiconductor wafers is used as a more effective alternative to wet chemical etching. The specific thin film layer deposited or grown on the semiconductor wafer to be plasma etched is typically covered with an apertured mask and placed in a gaseous environment. The gases directly above the wafer are ionized by RF energy to create a plasma and ions are accelerated out of the plasma to impact the semiconductor wafer. Etching of the portions of the thin film exposed through the masked apertures is accomplished as a function of the reaction of the material to the impact energy of the bombarding ions and the chemical activity of the gaseous environment.
In one commonly encountered situation the semiconductor wafer surface comprises a layer having one or more polysilicon films overlying a layer of a dielectric such as silicon dioxide. It is typically desired to etch through one polysilicon film exposing but not etching the dielectric or silicon dioxide layer, while also avoiding etching of the organic polymer of the mask. It is additionally desired to directionally limit the etching so as to etch preferentially downward without significant lateral etching. This avoids substantial bias undercutting of the polysilicon film under the edges of the masking layer. Such undercutting is to be avoided since it limits the device density on the semiconductor.
The mechanisms of plasma etching appear to involve both physical and chemical components. The gas pressure, RF frequency and power density as well as the gas composition determine whether the major reaction is physical or chemical. A chemically active gas such as a fluorine based compound is effective to attack and etch a polysilicon film but generates a good deal of chemical undercutting. Other gases by themselves, such as a chlorine or bromine containing Freon, applied as an etching gas in the presence of ion bombardment, will etch with more directionality, avoiding undercutting. This appears to be due to the fact that the ions bombarding the substrate have sufficient energy to produce a more anisotropic film profile. In the semiconductor structure utilizing doped or undoped polysilicon over silicon dioxide, or other dielectric, chlorine or bromine containing gases can, however, attack the organic polymer masking materials causing possible dimensional changes. Such gases can also etch into the polysilicon and dielectric films creating a negative sidewall slope eroding to corner pockets which fail to fill in with subsequent coatings. These pocket voids can lead to poor electric performance of the completed semiconductor chip.
The semiconductor producer must therefore compromise, settling either for the problems of undercutting induced by the presence of a fluorine based etching gas or the problems of photomask attack or poor etch selectivity and voids in the pockets of the underlying film caused by the presence of chlorine or bromine in the gases.